Don’t Use an Apostrophe to Make Names Plural

Never use an apostrophe to make a name plural. Apostrophes are for possessives.

The Joneses’ dinner was a success.

The Foxes’ house was beautiful.

The Alvarezes’ grandmother was delighted.

The Churches’ singing was heavenly.

The Ashes’ train derailed in the mountains.

How to Refer to More Than One In-Law

Now that we have the basics of making names plural, we can move on to trickier plurals: the in-laws. One thing that makes it tricky is that we call a group of them collectively “the in-laws,” but that’s not how you make them plural when you’re talking about a smaller group.

For example, if sister-in-law #1 and sister-in-law #2 are in the kitchen, you can safely gossip about those two sisters-in-law while you’re in the living room. They’ll never hear you over the music.

You make "in-law compounds" plural by making the noun part plural since the women are primarily your sisters and the "-in-law" part just further describes what kind of sisters they are. The same holds true for other in-laws. They are your

Brothers-in-law

Fathers-in-law

Mothers-in-law

The fact that we refer to them all as “in-laws” is just shorthand. Dictionaries call it a back formation (1).